Molten Salt Oxidation Research Articles

Reducing noble metal content in water electrolysis catalysts: A study on high-entropy oxides

This study presents the development of high-entropy oxide (HEO) catalysts based on Iridium-Ruthenium oxides (IrRuOx) for the oxygen evolution reaction (OER) in proton exchange membrane water electrolyzers (PEMWEs). By incorporating 3d transition metals such as Fe, Co, and Ni, we achieved catalysts with enhanced performance and stability. Using density functional theory (DFT) calculations, we predicted that HEOs possess a more favorable electronic structure for catalytic reactions. The synthesized rutile-type HEO catalyst (RuIrFeCoNi)O2, created via a molten salt oxidation method, exhibited a low overpotential of 261 mV and excellent cycling stability, maintaining a stable voltage of 1.73 V at 1 A cm−2 over 100 hours of electrolysis. This innovative approach not only reduces noble metal content but also leverages multimetallic synergistic effects to optimize the catalytic performance and stability, offering a promising avenue for the industrial application of acidic water electrolysis in PEMWEs.

Effect of feeding amount on tail gas and waste salt obtained from molten salt oxidation process of cation exchange resins

ABSTRACT The effective treatment of spent radioactive ion exchange resins generated during the operation of nuclear facilities is of great significance to reduce its potential hazards to the environment. This work innovatively provides a simple and low-cost molten salt oxidation (MSO) system includes air intake, feeding, waste salt discharge and exhaust gas treatment and analysis system for disposal of spent cation exchange resins (CERs). By studying three feeding amounts that injection of 25 g, 125 g and 1250 g CERs in 10 times under air conditions, the oxidation efficiency of CERs in MSO system was further evaluated. The scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) results indicated the sulfur can be trapped in the melt. Sulfur in functional group can be converted to Na2S and KLi(SO4) via Li2CO3-Na2CO3-K2CO3, which is proved by XRD patterns. The experimental results presented that degradation rates of CERs are 99.31%, 99.80%, and 99.91% in different feeding amounts, respectively. Furthermore, the volume reduction of residue gathered by feeding 1250 g CERs in 10 times was the highest, which can reach 9.19. Therefore, the bench-scale MSO system is a potential and promising treatment for spent CERs volume reduction.

Novel assessment of molten salt oxidation for cation exchange resin treatment: Effective neutralization of sulfurous gas with Li2CO3-Na2CO3-K2CO3 system

The conventional thermal treatment of cation exchange resin substantially releases sulfurous gases, causing significant equipment corrosion and air pollution. In contrast, the Li2CO3-Na2CO3-K2CO3 as an alkaline molten system effectively neutralizes sulfur gas and mitigates waste gas production inherent in thermal oxidation methods. In the molten salt oxidation process, the volume concentration of SO2 was reduced by 81.7 % compared to that in the traditional thermal oxidation process, and this method reduces the generation of hazardous gases such as CO, CH4, and C2H6. The integration of online gas mass spectrometry and phase stability diagrams for carbonate and sulfur interception products demonstrate excellent thermodynamic stability during the molten salt oxidation (MSO) process. Moreover, a more accurate assessment of the acid gas neutralization capacity of the molten salt system is provided, and the acid gas neutralization capacity of the Li2CO3-Na2CO3-K2CO3 carbonate system can reach 82.58 % at 800 °C. The predominant contributors to acid gas neutralization are Na2CO3 and K2CO3, as evidenced by waste salt composition and ternary phase diagrams. The stable presence of Li2CO3 throughout the MSO process contributes to the lowering of the melting point of the carbonate system to 393 °C.

The efficient oxidation of waste resins via ternary alkali carbonate: Mechanisms of the Fe3+/Co2+-catalysis for enhanced fixing organic sulfur, iron and cobalt

The development of a highly efficient and cost-effective strategy for the fixation of organic sulfur, as well as radioactive metal nuclides holds great significance in the treatment of organic contaminants. Herein, ternary molten carbonate (Li2CO3-Na2CO3-K2CO3) was used for treating simulated waste resins containing Fe3+ and Co2+ (Fe-Co-CERs). The FT-IR and XPS analysis showed that the significant conversion process of organic sulfur in molten Li2CO3-Na2CO3-K2CO3 was sulfonic acid group (−SO3-) → sulfuryl group (−SO2-) → sulfur bond (−S-). The TG-DTG curves and the activation energy (E) revealed Fe3+ and Co2+ possess self-catalysis during the oxidation of Fe-Co-CERs, wherein distinctly facilitated the decomposition of −SO3- and −S- structures. The produced SO2 was further absorbed by molten carbonate to generate sulfate, which was confirmed by the observed increase in SO42- and the corresponding decrease of CO32–. The conversion of LiNaSO4 to K3Na(SO4)2 revealed that Li2CO3 is to function as a sulfur catcher at temperatures below 400 °C, which then as a crucial heat transfer medium at temperatures above 400 °C. The predominant forms of Fe and Co at 700 °C were primarily Co8FeS8, FeS, and CoS, which underwent oxidation to form CoFe2O4 and CoSO4 at 800 °C. The retention rate of S, Fe and Co reached 94.36 %, 97.48 % and 98.35 %, respectively. This molten salt oxidation (MSO) strategy exhibits promising potential for the treatment of organic contaminants containing sulfur and multiple metal elements.

ZnO enhanced molten Na2CO3-K2CO3 system for simulated waste resins treatment: Efficient conversion and fixation of organic sulfur and iron(III)

The radioactive spent resins from pressurized water nuclear power plant always contain a significant proportion of 55Fe, which invariably leads to the release of radionuclides and acidic gases if not treated promptly. Herein, an efficient ZnO enhanced molten salt oxidation (MSO) method was established for the removal and fixation of organic sulfur and Fe in simulated radioactive resins containing Fe3+ (Fe-CERs). The existence of Fe3+ facilitates the reaction with O2 to form Fe2O3, which contributes to the fracture of organic skeleton. The comparative study was conducted to explore the impact of ZnO on the oxidation behavior of Fe and S at 600–800 °C. The introduction of ZnO caused the continuous formation of ZnS at 600–700 °C, and the conversion of ZnO at 800 °C indirectly promotes the retention rate of S (85.42 %). The potential conversion of Zn0.73Fe0.27S to ZnS may occur within the temperature range of 700 °C to 800 °C. Additionally, the predominant presence of carbon in residues at approximately 700 °C could exert a stimulating effect on the sulfidation process of Fe2O3 and inhibit the formation of SO2. The coexistence of Fe2O3, carbon and ZnO effectively diminishes H2S and SO2 amounts while further enhancing the fixation of S and Fe. The increase generation of K2SO4, FeS and Fe3O4 leads to a high formation rate of SO42- (79.36 %) and a great retention efficiency of Fe (96.23 %) at 800 °C. The proposed novel MSO method holds promising potential for applications in the treatment of organic contaminants.

Effect of Cr on the high-temperature corrosion of dissimilar AISI 904L and Inconel 625 joints employed in the coal-fired thermal power plant tubes

ABSTRACT The hot corrosion behaviours of 5-mm-thick plates of dissimilar AISI 904L and Inconel 625 weldments produced by continuous current gas tungsten arc welding process using Ni-Cr-Mo-enriched filler wires (ERNiCrMo-4 and ERNiCrCoMo-1) were investigated. Optical microscopy was used to examine the microstructure of these weldments. The corrosion performance of the weldments and base metals was determined in the mixture of K2SO4 + 60% NaCl molten salt (MS) and air oxidation conditions at 700°C for 50 cycles. The thermogravimetric plots derived the corrosion kinetics of the weldments. The microstructure and the elemental analysis of the scales formed on the corroded samples were evaluated by scanning electron microscopy and energy-dispersive spectroscopy. The phases developed on the oxide layers during the corrosion were analysed using X-ray diffraction. The fusion zones of ERNiCrCoMo-1 weldment and AISI 904L base metal were vulnerable to degradation in the MS environment, causing severe cracks and spallation compared to ERNiCrCoMo-1 weldment.

Detritiation of the Organic Material in the MSO Process

Tritium is a radioactive isotope of hydrogen, and due to its limited supply and related high price, it is necessary to recover it from tritiated materials via detritiation technologies. The number of tritiated materials arises during fusion. Tritium from fusion reactors is deposited in the outer layer of plasma-facing materials, which must be dealt with during maintenance or decommissioning processes. A second source of waste is laboratory equipment that could be contaminated with tritium. High-temperature treatment in an oxidation environment can achieve the release of tritium from metals or organic materials. The tritiated vapor is then captured in a series of water bubblers and reprocessed into pure T2. One of the high-temperature methods is molten salt oxidation (MSO), which uses high temperatures, alkaline salt, and an oxidative environment for flameless oxidation of different types of waste. This work aimed to simulate the detritiation processes of tritiated organic materials in MSO technology. First, a series of experiments with D2O absorbed in ion resins was conducted. The organic waste was decomposed within the molten salt, and the flue gas was measured to determine the oxidation efficiency. The D2O was captured in a series of water bubblers, and the water was then analyzed with attenuated total reflectance (ATR)–Fourier transform infrared spectroscopy (FTIR). The results showed that all deuterium in the form of D2O was caught in the first water bubbler. The capture efficiency ranged from 22.32% to 61.13%. The lower efficiency capture can be explained as D2O in water can form a HDO molecule and its correct determination is problematic. The second type of experiment was carried out with T2O with an activity of 851 Bq and with tritiated oil with an activity of 2495 Bq. The T2O was added to the set amount of ion resins and dosed into MSO. A peristaltic pump dosed the tritiated oil. The flue gas was measured to determine oxidation efficiency, and the T2O was captured within the water bubblers. The water was analyzed with liquid scintillation spectroscopy. The capture efficiency ranged between 76.42% to 97.87%. The results showed that this technology is suitable for the detritiation of tritiated organic materials.

Enhanced molten salt oxidation effect of nuclear grade cation exchange resin in lithium containing catalytic salt system

The Cation ion-exchange resins (CERs) can be effectively oxidized in molten Li2CO3-Na2CO3-K2CO3, but the high price of Li2CO3 requires further clarification of its necessity in the molten salt oxidation (MSO) process of CERs. In this paper, Li+ ions are introduced into CERs by ion exchange, and the direct oxidation of pure CERs and Li+ doped CERs and the MSO process of Li+ doped CERs in Na2CO3-K2CO3 (Li+ in CERs), pure CERs in Li2CO3-Na2CO3-K2CO3 (Li+ in molten salt) and pure CERs in Na2CO3-K2CO3 (without Li+) were compared. Thermo-gravimetric analyzer (TGA) and gas mass spectrometry data illustrate that the introduction of Li+ can increase the peak content of CO2 and SO2 and reduce the mass of residue. Fourier transform infrared spectroscopy (FT-IR) results reflect that the presence of Li+ can advance the temperature of resin skeleton destruction. Scanning electron microscope (SEM) displays the fracture degree of CERs is more severe in Li2CO3-Na2CO3-K2CO3 than other two systems. X-ray diffraction spectrum (XRD) and the content of carbonate and sulfate are used to analyze the composition of waste salt after MSO. Compared with other two systems, the content of sulfate produced by adsorption of SO2 and residual carbonate are the highest in Li2CO3-Na2CO3-K2CO3. Thermodynamic calculation using HSC chemistry 6.0 evidences that in Li2CO3-Na2CO3-K2CO3, Li2O produced by decomposition of Li2CO3 can provide oxygen ions, while Na2CO3 and K2CO3 can absorb SO2. Changing the oxidation temperature and oxidation time in different systems, the destruction and removal efficiency (DRE) of CERs are comprised. The DRE of Li+ in molten salt can achieve 99.99 % of CERs at a lower oxidation temperature and less oxidation time than other salt systems. In summary, the superiority of Li2CO3 in ternary salts is difficult to replace at present.

Characteristics of Molten Salt Gasification of Waste PVC

Molten salt oxidation is a robust thermal process with the inherent capability to catalytically oxidize the organic compounds while retaining the inorganic ingredients in salt bath. In the present study, molten salt gasification was used for the disposal of waste PVC. The characteristics of molten salt gasification of PVC under different temperatures and air equivalence ratios (ERs) on the gasification characteristics, chlorine retention efficiency, PCDD/F generation, and the distribution of heavy metals such as Cu, Pb, and Zn were investigated. The results showed that increasing the temperature and ER could effectively enhance the yield of gasification gas and carbon conversion efficiency. The highest gasification efficiency of 41.2% was achieved at 750 °C and ER = 0.4, with a gas yield of 0.442 Nm3/kg PVC. Molten carbonates showed an absorption and retention efficiency of more than 99.5% for chlorine under all conditions. Increasing temperature resulted in a significant reduction in the generation of PCDD/F. At 750 °C, the PCDD/F generation was less than 19 pg/g PVC with an I-TEQ of less than 1.4 pg/g PVC, and the ER had a minor effect on PCDD/F. During the molten salt gasification process, most of the heavy metals, such as Cu, Pb, and Zn, were retained in the salt bath.

Enhancing molten salt oxidation sustainability: thermodynamic insights for spent salt reuse and carbonate cycle replenishment

An optimized replenishment cycle for the molten salt oxidation method restored the SO2 adsorption capacity of the molten salt for the high-cycle treatment process, achieving over 99% oxidation efficiency and maintaining 80% sulfur interception.

Catalytic behavior of Mn during molten salt oxidation of cationic exchange resins in Li2CO3–Na2CO3–K2CO3 melt

Cationic exchange resins are used in nuclear power circuits to remove the nuclide ions to maintain the safe and efficient operation of nuclear power plants.

The conversion of organic sulfur and strontium into inorganic compounds during the molten salt oxidation of mixed resin

Molten salt oxidation (MSO) has the ability to capture sulfur and metal ions from radioactive mixed resin (MR). Nevertheless, the chemical conversion of sulfur (S) and strontium (Sr) in molten carbonate remains to be further investigated. Herein, MR doped with Sr2+ (Sr-MR) was used to replace spent mix resin containing radioactive 90Sr. The oxidation behaviors of MR and Sr-MR in ternary carbonate (Li2CO3-Na2CO3-K2CO3) were comparatively studied. Sr2+ could react with sulfur-containing species to form corresponding compounds in addition to the adsorption of SO2 by molten carbonate. The content of SO2 drops to 0 ppm at 800 °C, which indicates that the complete decomposition of sulfur-containing structures in Sr-MR and the efficient adsorption of SO2 by molten carbonate. Besides, Sr2+ could significantly promote the removal of organic sulfur, and the existence forms of strontium in molten salt are strontium sulfide (SrS, 350 °C), strontium oxide (SrO, 450 °C) and strontium carbonate (SrCO3, ≥ 550 °C). The presence of quaternary ammonium group (-N+(CH3)3) in Sr-MR indirectly stimulated the formation of sulfates from sulfur-containing compounds. The residual amounts of S and C in Sr-MR decreased to 1.44 % and 6.89 % at 650 °C, which were lower than that of pure MR under the same conditions. The proposed MSO system had excellent retention effects of Sr (98.51 %) and S (47.66 %) at 800 °C for 2.0 h, which provides a new perspective on the disposal of organic wastes containing sulfur radioactive metal ions.

An ideal method for efficiently intercepting sulfur and cesium during molten salt oxidation of cation exchange resins doped with cesium

An ideal method for efficiently intercepting sulfur and cesium during molten salt oxidation of cation exchange resins doped with cesium

Study on molten salt oxidation process of simulated Co doped cation exchange resins

Cation exchange resins (CERs) are widely applied to purify waste liquids generated during the operation of nuclear reactors. The radioactive nuclides 60Co and 58Co are important corrosion activation products in reactor cooling water. In this study, the simulated Co doped CERs were oxidized with ternary carbonate. According to the thermogravimetric analysis (TG), the decomposition of Co doped CERs includes three processes: 1. Elimination of the osmotic water; 2. Pyrolysis of sulfonic acid group; 3. Destruction of styrene–divinylbenzene copolymer. The X-Ray Diffraction (XRD) patterns indicate that sulfur mainly exists in the form of sulfate in waste salt. The Co2+ undergoes the path of CoS2 → Co3O4 with the increase of temperature and the transition point is 650 °C. Combined with Fourier Transform Infrared Spectrometer (FT-IR) spectra and X-ray Photoelectron Spectroscopy (XPS) analysis, sulfonic acid groups begin to decompose at 350 °C. During the molten salt oxidation process, most of the sulfur in sulfonic acid groups is entrapped by carbonate as the form of sulfate, and a little of which remains as sulfone group, sulfoxide group and sulfur bridge in residue. When the resins are oxidized at 800 °C, the retention rate of Co2+ is 97.3%, indicating that the molten salt oxidation can effectively remain Co2+ and convert it into a more stable substance.

Effect of copper ions on transformation of organic sulfur in cationic exchange resins in Li2CO3–Na2CO3–K2CO3 molten-salt system

Cationic exchange resins (CERs) were applied for purification and clarifying process of radioactive wastewater in nuclear industry, which was a kind of sulfur-containing organic material. Molten-salt oxidation (MSO) method can be applied for the treatment of spent CERs and the absorption of acid gas (such as SO2). The experiments about the molten salt destruction of the original resin and Cu ions doped resin were conducted. The transformation of organic sulfur in Cu ions doped resin was investigated. Compared with the original resin, the content of tail gas (such as CH4, C2H4, H2S and SO2) released from the decomposition of Cu ions doped resin was relatively high at 323–657 °C. Sulfur elements in the form of sulfates and copper sulfides were fixed in spent salt through XRD analysis. The XPS result revealed that the portion of functional sulfonic acid groups (-SO3H) in Cu ions doped resin was converted into sulfonyl bridges (-SO2-) at 325 °C. With the enhancement of temperature, sulfonyl bridges (-SO2-) were further decomposed to sulfoxides sulfur (-SO-) and organic sulfide sulfur. The destruction of thiophenic sulfur to H2S and CH4 was prompted by copper ions in copper sulfide. Sulfoxide were oxidized to the sulfone sulfur in molten salt. Sulfones sulfur consumed by reduction of Cu ions at 720 °C was more than it produced by oxidation of sulfoxide through XPS analysis, and the relative proportion of sulfone sulfur was 16.51%.

Efficient fixation of Co2+ and organic sulfur from cation exchange resins into CaO enhanced molten Li2CO3-Na2CO3-K2CO3 system

The radioactive spent resins produced by the nuclear power plant contain a certain amount of 60Co. These spent resins are easy to generate volatile substances during the heat treatment process, resulting in the leakage of radionuclide. The molten salt oxidation (MSO) can effectively prevent the volatilization of Co, and can facilitate the removal of sulfur exists in the resins. In this work, ternary carbonate (Li2CO3-Na2CO3-K2CO3) was used as molten salt system, and CaO was used as a sulfur pollutant capture agent. The non-radioactive CoCl2 served as simulate radioisotopes, and the Co2+ doped cation exchange resins (Co-CERs) were obtained by the ion exchange method. The introduction of Co2+ promoted the further oxidation of the thermally stable structure SC, and the harmful gases SO2 and H2S generated during the MSO process were effectively absorbed by carbonate with the participation of CaO. The retention rate of S could reach 87.79 % with the addition of CaO at 800 °C, which was 1.81 % higher than that without CaO. The test results of X-ray diffraction (XRD), scanning electron microscope (SEM) and inductively coupled plasma mass spectrometry (ICP-MS) confirmed that the addition of CaO promoted the formation of sulfide and thermally stable CoSO4. The retention rate of Co2+ in waste salt reached 97.36 % at 800 °C. The CaO enhanced molten carbonate system can realize the efficient treatment of Co-CERs and possess potential applications on organic contaminant treatment.

Possibility of processing tungsten dust waste from the nuclear fusion process by a combination of several technologies

The processing of tungsten dust waste from the nuclear fusion process is being considered for the EU DEMO within the Workpackage Safety and Environment in the EUROfusion programme.A multi-step procedure with a combination of several independent technologies is proposed. Specific technologies are gradually being proposed, however their combination has not been tested yet. Removal of tritium from tungsten dust by a Molten Salt Oxidation technology (MSO) is investigated while, simultaneously, tungsten is retained in the molten salt and tritium is trapped in the form of condensed tritium water. Tungsten salts are concentrated in several stages by a combination of chemical methods, settling, and centrifugation. The dissolution and concentration procedures alternate in a defined manner. The concentrate is gradually enriched with tungsten and tungsten salts. The molar volume of tungsten progressively increases in the concentrated salt suspension.Further removal of certain impurities, decomposition of salts, and the increase of the tungsten concentration in the product can be achieved by means of Induction Melting or Induction Heating in a Cold Crucible (IMCC or IHCC) in a suitable atmosphere. The final stage is the reprocessing of the concentrated tungsten product from the dust form into a compact piece of material. Induction Heating technology will enable its sintering, compaction, and reduction of porosity.

Influence of oxygen equivalent on molten salt oxidation efficiency of mixed resin in Li2CO3-Na2CO3-K2CO3 melt

The disposal of spent radioactive ion exchange resin generated during the operation of nuclear facilities has always been a conundrum. The molten salt oxidation (MSO) for the treatment of mixed resin (MR) shows obvious superiority. In this work, ternary carbonate (Li2CO3-Na2CO3-K2CO3) and MR was used as the molten salt system and the oxidation target, respectively. The oxidation behavior of MR was analyzed by varying the temperature and oxygen equivalent during the MSO process. By studying the effect of different oxygen equivalents on the oxidation efficiency, the oxygen equivalent of 125% could make the oxidation efficiency of MR reach 99.99% at 800 °C. The composition of C, N and S containing exhaust gas produced through MSO process of MR with temperature were almost consistent with the simulation results. The exhaust gas was successfully adsorbed by molten carbonate to produce nitrate and sulfur compounds. The carbonate has good absorption to harmful gases such as SO2, CO, NO, etc. The content of SO2 from the highest 0.32% to 0, and 71.23% of sulfur in MR was trapped by molten carbonate as the form of sulfate. This work has important implications for reducing the potential harm of radioactive waste resin to the environment.

Degradation mechanisms of organic compounds in molten hydroxide salts: a radical reaction yielding H2 and graphite.

Molten salts are used in various waste treatments, such as recycling, recovery or making inert. Here, we present a study of the degradation mechanisms of organic compounds in molten hydroxide salts. Molten salt oxidation (MSO) using carbonates, hydroxides and chlorides is known for the treatment of hazardous waste, organic material or metal recovery. This process is described as an oxidation reaction due to the consumption of O2 and formation of H2O and CO2. We have treated various organic products, carboxylic acids, polyethylene and neoprene with molten hydroxides at 400 °C. However, the reaction products obtained in these salts, especially carbon graphite and H2 without CO2 emission, challenges the previous mechanisms described for the MSO process. Combining several analyses of the solid residues and the gas produced during the reaction of organic compounds in molten hydroxides (NaOH-KOH), we demonstrate that these mechanisms are radical-based instead of oxidative. We also show that the obtained end products are highly recoverable graphite and H2, which opens a new way of recycling plastic residues.

The effect of air on oxidation decomposition of uranium-containing cationic exchange resins in Li2CO3-Na2CO3-K2CO3 molten-salt system.

With the development of nuclear energy, spent cationic exchange resins after purification of radioactive wastewater must be treated. Molten-salt oxidation (MSO) can minimize the disposal content of resins and capture SO2. In this work, the decomposition of uranium-containing resins in carbonate molten salt in N2 and air atmospheres was investigated. Compared to N2 atmosphere, the content of SO2 released from the decomposition of resins was relatively low at 386-454 °C in an air atmosphere. The SEM morphology indicated that the presence of air facilitated the decomposition of the resin cross-linked structure. The decomposition efficiency of resins in an air atmosphere was 82.6% at 800 °C. The XRD analysis revealed that uranium compounds had the reaction paths of UO3 → UO2.92 → U3O8 and UO3 → K2U2O7 → K2UO4 in the carbonate melt, and sulfur elements in resins were fixed in the form of K3Na(SO4)2. The XPS result illustrated that peroxide and superoxide ions accelerated the conversion of sulfone sulfur to thiophene sulfur and further oxidized to CO2 and SO2. Besides, the ion bond formed by uranyl ions on the sulfonic acid group was decomposed at high temperature. Finally, the decomposition of uranium-containing resins in the carbonate melt in an air atmosphere was explained. This study provided more theoretical guidance and technical support for the industrial treatment of uranium-containing resins.